In a conventional cutting and welding system, plasma arc torches are supported on a frame bridge and carriage system. Drive means moves the plasma arc torches along the X - Y - Z axis during the cutting or welding of a workpiece positioned beneath the torch. A microprocessor controller, which usually includes a numerical control operating system, provides precise control over the position, movement, and acceleration of the torch to enable precision cutting and welding of the workpiece.
The plasma arc torches used in such systems usually are water assisted, although in some systems, the torches include an annular gas sheath generated around the discharged plasma instead of the usual annular jet of water. The torches also include water cooling circuits to prevent the torch from overheating during operation.
During system operation, a power supply generates a cutting current to the torch electrode, and generates an electrical arc extending from the electrode through a bore of the nozzle assembly of the torch into contact with the workpiece positioned beneath the nozzle assembly. A flow of cutting gas is generated between the electrode and the nozzle assembly, and the gas contacts the electrical arc to form a plasma flow through the bore.
During initial setup for system operation, the system operator presets the amount of cutting current and the amount of gas and water flow into the torch. The operator then manually adjusts the cutting current to accommodate load changes which occur through process parameter changes such as changes in plate thickness, changes in plasma gas flow, changes in the distance from the plate to the torch, and changes in the gas and water flows into the torch. In this system, each plasma arc torch includes a separate control for manually varying the cutting current and the gas and water flow into the plasma arc torch. A unitary control for several torches is not advantageous because of the inherent difficulty in balancing the gas, water and power between the different torches.
In many conventional systems, the water, gas and power controls are manually operated during system operation to enable the operator to set the flows and cutting currents on an as-needed basis. The controls usually are efficient and simple in design to reduce the overall cost of the system. In one conventional cutting and welding system marketed as a CM-300 Gantry Shape Cutting Machine, manufactured by ESAB Welding Products, Inc. of Florence, S.C., the gas and water pass through conventional rotameters and valve mechanisms associated with the rotameters. The operator visually measures the flow by reading indicia positioned on the side of the tapered measuring tubes common to the rotameters. The valves are adjusted accordingly.
The rotameters and the valves are usually positioned closely adjacent the torches. In one system, the tubes and valves are positioned in a flow control box mounted on the frame supporting the torches so that the tubes and valves move with the torches during cutting or welding, making manual adjustment of the gas and water flows during system operation difficult.
A more complex valve mechanism could be substituted for the conventional rotameters and valves. For example, a closed loop control system could be used for measuring water and gas output and complex valve mechanisms then could be adjusted for varying flow through the valves. However, such valves are complex and expensive, and require sophisticated control and software technology, making their use in a plasma arc torch welding and cutting system prohibitive.
In such manual water flow control systems, standard flow control valves also are positioned adjacent the water pumps to provide on-off control of water flowing into the torch. Typically, the valves are electrically actuated, and after extended use, the valves may become stuck in an open or closed position. If the torch is operated with a closed valve, the torch may overheat. Thus, operators often electrically bypass the flow control valve when the valve is open, hoping that the valve remains in the open position. If the valve closes, and the operator starts system operation under the belief that the valve is open, then the plasma arc torch will overheat, resulting in a damaged torch.
Additionally, in such cutting systems, a power supply provides a cutting current to the electrode to create a plasma flow through the bore to a workpiece positioned beneath the nozzle assembly. In one common power supply manufactured by ESAB Welding Products, Inc. of Florence, S.C., under the Serial designation ESP-400, the cutting current of this system is precisely regulated by an analog reference voltage signal.
The cutting current is a linear function of the analog reference voltage signal. The analog signal is adjusted by manually turning a small control knob, which varies the analog signal and varies the cutting current. During some cutting and welding operations, process parameters such as the thickness of the plate, the distance from the nozzle to the plate, and the operating temperature may change. As a result, the cutting current may vary as the process parameters change, and the operator must adjust the control knob to maintain a constant current output to the torch. Constant operator adjustment may result in some operator error, and the loss of cut or weld quality.